The present invention relates in general to detecting objects and/or areas. More particularly, the invention provides a method and system for adaptive variable true time delay beam forming. Merely by way of example, the invention is described as it applies to a phased array antenna, but it should be recognized that the invention has a broader range of applicability.
A phased array antenna has been widely used for communications and radar systems. The phased array antenna usually does not mechanically steer antenna directions, and can provide rapid beam scanning. The directivity of the phased array antenna can be achieved by properly adjusting the relative phases between signals transmitted or received by different antenna elements. These antenna elements can reinforce the transmitted or received radiation in a desired direction.
FIG. 1 is a simplified diagram for a conventional phased array antenna. An arrival signal 140 with a center wavelength λ0 arrives at an array of antenna elements 110. The angle of arrival is θ0. Phase shifters 120 are applied to the outputs of the antenna elements 110 and generate phase delayed signals. The sum of the phase delayed signals forms an output beam 130. The phase shifters 120 are usually adequate for forming the output beam 130 if the 3 dB bandwidth of the arriving signal 140 is narrow and the scan angle θ0 is small. Otherwise, a time delay circuit is usually needed for beam formation. For example, the time delay is needed when                     B        >                  0.886                      τ            0                                              (                  Equation          ⁢                                          ⁢          1                )                                          τ          0                =                                            Nd              x                        ⁢            sin            ⁢                                                  ⁢                          θ              0                                                          f              0                        ⁢                          λ              0                                                          (                  Equation          ⁢                                          ⁢          2                )            
where B is the 3 dB bandwidth of the arriving signal 140, and τ0 is the total time delay across the array of antennal elements 110. Additionally, f0 is the center frequency of the arriving signal 140, N is the total number of antenna elements 110, dx, is the distance between two adjacent antenna elements 110, and θ0 is the angel of arrival. As another example, if the total time delay, τ0, across the array of antenna elements 110 is greater than the reciprocal of the 3 dB bandwidth, the time delay is usually needed for beam forming.
In certain beam forming applications, the received or transmitted signals need to maintain phase continuity and avoid any abrupt phase transition. Phase continuous variable true time delay circuits are usually used. The phase continuous variable true time delay circuits can be implemented by switching in and out of a plurality of RF cables or optical fibers of different lengths. But during the switching of cables, an abrupt phase transition may be introduced into the processed signals. As the size of the antenna aperture and the number of antenna elements become large, testing and calibration of the entire antenna system also become difficult.
Hence it is highly desirable to improve techniques for adaptive variable true time delay beam forming.